Because by allowing corporations to influence who gets elected, corporations will influence what legislation gets passed or blocked that is in their best financial interests, not necessarily in the interests on the individual. Think Big Pharma and Big Insurance Barons.
This will become very evident in the coming mid-term elections and could cripple health care reform in this country. The" land of the free and the brave" suddenly has become even more the land of big business.
Here is the MSNBC article on the ruling and which justices voted in favor of this change. Note that the female justices opposed the ruling.